Code No. P0138: Oxygen Sensor (Rear) Circuit High Voltage

OPERATION

• The sensor signal is inputted to the engine-ECU (terminal No. 40) from the oxygen sensor (rear) output terminal (terminal No. 3).
  
• The oxygen sensor (rear) (terminal No. 4) is earthed with engine-ECU (terminal No. 41).
  

FUNCTION

• The oxygen sensor (rear) converts the concentration of oxygen in the exhaust emission into a voltage and inputs the signal to the engine-ECU.
  
• When the air-fuel ratio is richer than the theoretical air-fuel ratio, the oxygen sensor (rear) outputs a voltage of about 1 V. When it is leaner than the theoretical air-fuel ratio, it outputs a voltage of about 0 V.
  
• In response to the signal, the engine-ECU controls the fuel injection amount so that the air-fuel ratio can be equivalent to the theoretical air-fuel ratio.
  

TROUBLE JUDGMENT

• 2 seconds or more have passed since the engine starting sequence was completed.
  
• Oxygen sensor offset voltage is 0.4 - 3.8 V.
  

• Oxygen sensor (rear) output voltage has continued to be 1.2 V or higher for 2 seconds.
  

PROBABLE CAUSES

• Failed oxygen sensor (rear)
  
• Short circuit in oxygen sensor (rear) heater circuit or loose connector contact
  
• Failed engine-ECU
  

	Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).

	Go to Step 2 .

	Go to Step 3 .

	Repair or replace the connector.

	Go to Step 4 .

	Repair or replace the connector.

	Go to Step 5 .

	Repair the damaged harness wire.

	Go to Step 6 .

	Repair the damaged harness wire.

	Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).

	Replace the engine-ECU.